1. Field of the Invention
This invention relates to spark ignition systems for internal combustion engines; and more particularly to a spark ignition system including a capacitive discharge system and a core-coil assembly which improves performance of the engine system and reduces the size of the magnetic components in the spark ignition transformer in a commercially producible manner.
2. Description of the Prior Art
In a spark-ignition internal combustion engine, a flyback transformer is commonly used to generate the high voltage needed to create an arc across the gap of the spark plug and cause an ignition event, i.e., igniting the fuel and air mixture within the engine cylinder. The timing of this ignition spark event is critical for best fuel economy and low exhaust emission of environmentally hazardous gases. A spark event which is too late leads to loss of engine power and efficiency. Correct spark timing is dependent on engine speed and load. Each cylinder of an engine often requires different timing for optimum performance. Different spark timing for each cylinder can be obtained by providing a spark ignition transformer for each spark plug.
To improve engine efficiency and alleviate some of the problems associated with inappropriate ignition spark timing, some engines have been equipped with microprocessor-controlled systems which include sensors for engine speed, intake air temperature and pressure, engine temperature, exhaust gas oxygen content, and sensors to detect "ping" or "knock".
A disproportionately greater amount of exhaust emission of hazardous gases is created during the initial operation of a cold engine and during idle and off-idle operation. Studies have shown that rapid multi-sparking of the spark plug for each ignition event during these two regimes of engine operation reduces hazardous exhaust emissions. Accordingly, it is desirable to have a fast cycling spark ignition system.
Engine misfiring increases hazardous exhaust emissions. Numerous cold starts without adequate heat in the spark plug insulator in the combustion chamber can lead to misfires, due to deposition of soot on the insulator. The electrically conductive soot reduces the voltage increase available for a spark event. A spark ignition transformer which provides an extremely rapid rise in voltage can minimize the misfires due to soot fouling.
A coil-per-spark plug (CPP) ignition arrangement in which the spark ignition transformer is mounted directly to the spark plug terminal, eliminating a high voltage wire between the conventional engine coil and spark plug, is gaining acceptance as a method for improving the spark ignition timing of internal combustion engines. One example of a CPP ignition arrangement is disclosed in U.S. Pat. No. 4,846,129 to Noble (hereinafter "the Noble patent"). The physical diameter of the spark ignition transformer must fit into the same engine spark plug well in which the spark plug is mounted. To achieve the engine diagnostic goals envisioned in the noble patent, the patentee discloses an indirect method utilizing a ferrite core. Ideally the magnetic performance of the spark ignition transformer is sufficient throughout the engine operation to sense the sparking condition in the combustion chamber.
To achieve the spark ignition performance needed for successful operation of the ignition and engine diagnostic system disclosed by Noble and, at the same time, reduce the incidence of engine misfire due to spark plug soot fouling, the spark ignition transformer's core material: (i) must have certain magnetic permeability; and (ii) must have low magnetic losses. In a capacitive discharge (CD) system, very fast rise times and rapid energy transfer are critical. The magnetic core material must be capable of high frequency response with low loss. The combination of these required properties narrows the availability of suitable core materials. Considering the target cost of an automotive spark ignition system, possible candidates for the core material include silicon steel, ferrite, and iron-based amorphous metal. Conventional silicon steel routinely used in utility transformer cores is inexpensive, but its magnetic losses are too high. Thinner gauge silicon steel with lower magnetic losses is too costly. Ferrites are inexpensive, but their saturation inductions are normally less than 0.5 Tesla (T) and Curie temperatures at which the core's magnetic induction becomes close to zero are near 200.degree. C. This temperature is too low considering that the spark ignition transformer's upper operating temperature is assumed to be about 180.degree. C. Iron-based amorphous metal has low magnetic loss and high saturation induction exceeding 1.5 T, however it shows relatively high permeability. An iron-based amorphous metal capable of achieving a level of magnetic permeability suitable for a spark ignition transformer is needed. Using this material, it is possible to construct a toroid design coil which meets required output specifications and physical dimension criteria. The dimensional requirements of the spark plug well limit the type of configurations that can be used. Typical dimensional requirements for insulated coil assemblies are less than 25 mm in diameter and less than 150 mm in length. These coil assemblies must also attach to the spark plug on both the high voltage terminal and outer ground connection and provide sufficient insulation to prevent arc-over from the coil to other engine components. The outer ground connection can be made via a return from the engine block, as in typical coil-per-plug systems. These must also be the ability to make high current connections to the primary coil windings typically located on top of the coil.